GXT provides the widgets to draw many user interfaces, but we don't do the work of implementing features like this. If you can find a Java library that can be used in GWT, you may be able to run that, then find and restyle the added/removed rows as in your picture. But there is no built-in support to do this for you.

Think of it like this - GXT provides the tools with which one can build something like GitHub or FishEye (webapps that allow diffing code), but doesn't actually build the functionality for you.